A mechanical apparatus for performing movements like those of the human being, using electrical or magnetic operations, is termed a “robot”. The etymology of the term robot is said to be “ROBOTA” (slave machine) of the Slavonic language. The robot started to be used in this nation extensively in later sixties. Most of the robots used were industrial robots, such as manipulator and a transport robot, aimed to automate the production operations in plants or to perform unmanned operations in plants.
In recent years, developments of utility robots, supporting the human life as a partner to the human being, that is, supporting the human activities in various aspects in our everyday life, are progressing. In distinction from a crawler type robot, or a robot walking on four or six legs, the robot erected and walking on two legs is labile and experiences increasing difficulties in posture or walking control. However, the robot erected and walking on two legs is superior in being able to realize flexible motion operations, such as by coping with walking surfaces presenting protuberances or recesses, as when the robot walks on an untrimmed terrain, with walking surfaces presenting obstacles on a working route, or with non-continuous walking surfaces, as when he robot has to go up or down a stairway or a ladder.
The legged mobile robot, simulating the living mechanism or movements of the human being, is termed a “humanoid” or a “humanoid robot”. The humanoid robot is able to support the human life, that is, able to support human activities in various aspects in our everyday life, such as in our living environment.
The working space or the living space of the human being is mostly formed to the bodily mechanism and action patterns proper to the human being, erected and walking on two legs, while presenting many obstacles to the motion of the mechanical system of the current state of the art, having wheels or other driving devices as motion means. Hence, if such mechanical system, that is, a robot, is to perform various human operations to take the place of the human being and to adapt itself to our living space, it is desirable that the range of possible motion of the robot is approximately equal to that of the human being. This accounts for general expectations entertained for putting the legged mobile into practical use.
A great variety of technologies pertinent to posture control or stable walking of the two-legged mobile robot have already been proposed. The stable “walking” may be defined as “motion on legs without falldown”. The stable posture control for a robot is crucial for avoiding the falldown of the robot because the falldown means interruption of the operations being carried out by the robot and considerable labor and time are needed for the robot to rise from the falldown state to reinitiate the operations. Moreover, such falldown is likely to inflict fatal damages to the robot itself or to a counterpart object against which the robot collides in the course of falldown thereof.
The majority of the proposals for stable posture control of the robot and prevention of falldown during walking use the ZMP (zero moment point) as the criterion for verifying the walking stability. The criterion for verifying the stability by ZMP is based on the ‘D' Aembert's principle’ that the force of gravity, the force of inertia and the moment thereof in a direction from the floor surface to the walking system, counterbalance the force of reaction from the floor and the moment thereof in a direction from the floor surface to the walking system. As a conclusion of the mechanical inference, there is a point of zero pitch axis moment and zero roll axis moment inwardly of a support polygon defined by the floor contact point of the foot sole and the floor surface, that is, a ZMP stable area (see, for example, Miomir Vukobratovic, “Legged Locomotive Robots” (translated by Ichiro KATO et al., “Walking Robot and Artificial Leg”, published by NIKKAN KOGYO SHIMBUN-SHA).
In sum, the ZMP criterion is that if, in all instants of walking, the ZMP exists inwardly of a support polygon defined by the foot and the floor surface, and there acts a force in a direction the robot thrusts the floor surface, the robot can walk in stability without falldown (i.e. without the robot body performing rotating motion).
The generation of a two-legged walking pattern, based on the ZMP criterion, has a merit that the touchdown point of the foot sole can be previously set and that account may readily be taken of kinematic constraint conditions of the foot sole in keeping with the floor surface profile. Moreover, using the ZMP as the criterion for verifying the stability means handling the trajectory rather than the force as a target value for kinematic control, and hence raises technical feasibility.
For example, the legged mobile robot is able to perform stabilized walking in such a manner that a point on a floor surface corresponding to the zero ZMP coincides with a target value (see, for example, the Japanese Laid-Open patent Publication H-5-305579).
Moreover, the legged mobile robot may be constructed so that the ZMP will be inside a support polyhedron (polygon) or so that the ZMP will be at a distance with a certain allowance from an end part of the support polygon at the time of touchdown or clearing the floor (see for example the Japanese Laid-Open patent Publication H-5-305581). In this case, there is an allowance of the ZMP by a certain preset distance, even under occurrence of interference, thus improving stability of the robot body during walking.
The walking speed of the legged mobile robot may also be controlled by a ZMP target position (see, for example, the Japanese Laid-Open Patent Publication H-5-305583). That is, pre-set walking pattern data are used, and the joints of the leg parts are driven so that the ZMP coincides with a target position. The tilt of the upper body is detected and the walking pattern data emitting rate is changed depending on the detected value. In case the robot treads on an unknown irregular surface and thus leans forward, the emitting rate is quickened to recover the posture. Since the ZMP is controlled to the target position, the emitting rate may be changed without any inconvenience if the emitting rate is changed during the time both leg units of the robot are in the stance position.
On the other hand, the touchdown position of the legged mobile robot may be controlled by the ZMP target position (see for example the Japanese Laid-Open patent Publication H-5-305585). That is, in the legged mobile robot, described in this Patent Publication, the deviation between the ZMP target position and the actually measured position is detected. In order to eliminate this deviation, one or both of the leg units are driven, or the moment about the ZMP target position is detected and the leg units are actuated to reduce the moment to zero such as to realize stable walking.
The leaning posture of the legged mobile robot may also be controlled by the ZMP target position (see for example the Japanese Laid-Open patent Publication H-5-305586). That is, the moment about the ZMP target position is detected and, if such moment is produced, the leg units are driven such as to reduce the moment to zero, in order to effect stable walking.
The posture stabilizing control of the robot, employing the ZMP as the criterion for verifying the stability, resides in searching a point of zero pitch axis moment and zero roll axis moment within a support polygon defined by the touchdown point of the foot sole and the floor surface.
However, as a result of the a priori validation by the present inventors, it has become clear that, when the robot performs high-speed leg movements, not only the moments about the pitch axis and about the roll axis of the robot body, but also the moment about its yaw axis, is produced.
FIG. 1 shows an illustrative relationship between the walking speed [second/step] of the two-legged mobile robot and the moment [Nm] generated in the yaw axis direction. As may be seen from this figure, the shorter the time needed per step of the legged mobile robot, that is, the higher the walking speed, the larger becomes the moment about the yaw axis to a marked extent.
Such moment about the yaw axis produces an action of causing the swinging movement of the robot body, sooner or later, producing a slip about the yaw axis between the foot sole of the robot and the floor surface to affect the walking stability appreciably or otherwise proving a hindrance in the realization of the stable and accurate legged operations in an expected manner. If the factor of the moment about the yaw axis becomes excessive, robot falldown may be produced to destruct the robot body and an article against which the robot falls.
On the other hand, the legged mobile robot is made up by a plurality of part groups, such as left and right upper limbs, left and right lower limbs and a body trunk, and includes a large variety of degrees of freedom of the joints. It is however not known precisely how the plural parts are to operate in concert in order to cancel out the moment errors applied to the robot body about the roll, pitch and yaw axes during its movements, such as walking.
In the posture stabilizing control for the robot, employing ZMP as a criterion for verifying the stability, a point with the zero pitch axis moment and the zero roll axis moment is searched within a support polygon defined by the floor touch point of the foot sole and the floor surface. However, in case both leg units have cleared the floor surface, such as when the robot has jumped or descended from an elevated object, there is no support polygon, such that the conventional technique of controlling the ZMP to be within the inside of the support polygon cannot be applied.
It is an object of the present invention to provide a method and an apparatus optimum for controlling the operation of the robot apparatus, and a computer program, in which the target trajectory for each part on the robot body may be corrected to compensate an unknown external moment or an unknown external force.
It is another object of the present invention to provide a method and an apparatus optimum for controlling the operation of the robot apparatus, and a computer program, in which the moment error applied to the robot body about the roll, pitch and yaw axes during the robot's movements, such as walking, may be canceled out with advantage by concerted operations of the respective part groups making up the robot body.
It is another object of the present invention to provide a method and an apparatus optimum for controlling the operation of the robot apparatus, and a computer program, in which the moment error applied to the robot body about the roll, pitch and yaw axes may be canceled out with advantage even when the apparatus clears the floor surface, such as during jumping or descending from an elevated place.